Fluid compositions which undergo a change in apparent viscosity in the presence of an electrical field are commonly referred to as electrorheological fluids. Electrorheological fluids normally are comprised of particles dispersed within a carrier fluid and in the presence of an electrical field, the particles become polarized and are thereby organized into chains of particles within the fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the overall fluid and in the absence of an electric field, the particles return to their unorganized or free state and the apparent viscosity or flow resistance of the overall fluid is correspondingly reduced.
An electrorheological fluid composed of a non-conductive solid dispersed within an oleaginous fluid vehicle is described in U.S. Pat. No. 3,047,507. The compositions contain a minimum amount of water and a minimum amount of a surface active dispersing agent and the non-conductive solid consists of finely divided particles having an average diameter of from about 0.1 to about 5 microns.
U.S. Pat. No. 4,702,855 discloses electrorheological fluids consisting of an aluminum silicate solid dispersed within a fluid medium wherein the aluminum/silicate atomic ratio on the surface of the aluminum silicate is in the range of 0.15 to 0.80. The aluminum silicates may be either amorphous or crystalline and may contain contaminants such as Fe.sub.2 O.sub.3, TiO.sub.2, CaO, MgO, Na.sub.2 O, and K.sub.2 O. The electrorheological fluids may optionally contain an effective quantity of an appropriate dispersing agent.
Electrorheological fluids containing a suitable quantity of finely divided, particulate conductive materials for use within an alternating-field chucking device are disclosed in U.S. Pat. No. 3,385,793. The conductive material may be a metal such as copper, iron, aluminum or zinc. The conductive material is incorporated into an electrorheological fluid consisting of a particulate solid dispersed in an oleaginous vehicle which may optionally contain surface active agents or activators.
U.S. Pat. No. 4,645,614 discloses an electrorheological fluid consisting of aqueous silica gel dispersed within silicone oil and containing an amino functional or silicon functional polysiloxane having a molecular weight above 800 as a dispersant. The dispersant is present in a concentration of from 1 to 30 percent by weight based on the silica gel particles.
As further background, it should be noted that in order for particles suspended in a fluid medium to appear transparent, the index of refraction of the fluid n.sub.1 must match the index of refraction of the particle n.sub.2 (i.e., n.sub.1 .apprxeq.n.sub.2) so that little or no light scattering takes place at the fluid-particle interfaces. Since the index of refraction n of a material is related to the permittivity .epsilon. or dielectric constant K of the material by Maxwell's relation: ##EQU1## where .epsilon..sub.o is the permittivity of free space, the particles and fluid in a transparent suspension are expected to have matching or identical permittivities.
However, in order for a material to polarize and respond as an electrorheological fluid, the particle and the fluid must have different permittivities. The polarizability .beta. of a particle in a fluid medium is given by: ##EQU2## where .epsilon..sub.1 is the permittivity of the fluid and .epsilon..sub.2 is the permittivity of the particle. If .epsilon..sub.2 =.epsilon..sub.1, then .beta.=0, no polarization occurs, and no electrorheological effect is observed. A highly transparent suspension would be expected to have .epsilon..sub.2 =.epsilon..sub.1 and would thus not be expected to work as an electrorheological fluid.
In certain applications, it is desirable to utilize an electrorheological fluid which is optically transparent so that the inner working and electrode structures of a device may be observed during operation. For example, in devices that rely on light transmission, it is necessary for light to pass through the electrorheological fluid in order to be properly detected by sensory equipment. A transparent electrorheological fluid could also be utilized in conjunction with colored particles to map out flow patterns or detect regions of electrical breakdown within the electrorheological fluid. An electrorheological fluid is therefore needed which exhibits sufficient optical transparency such that the inner working and electrode structures of an electrorheological device may be observed by the human eye.